In subjects with chronic balance disorders, approaches to medical treatment based on drugs and surgery can be effective in stabilizing the pathological processes that cause the disorders. In such subjects, these approaches can sometimes stabilize but seldom fully resolve the underlying pathological manifestations of the balance problem. Once the underlying pathological processes are medically stable however, rehabilitation exercises frequently prove effective in reducing many of the disabling symptoms and functional problems associated with chronic balance disorders. Hence, effective treatment of chronic balance disorders typically employs combinations of medical and rehabilitation exercise treatments.
In selecting medical treatments most likely to stabilize underlying pathological processes, clinicians first determine the location, nature, and extent of the underlying pathological process. To make pathological determinations, clinicians typically rely on the results of the subject history and physical examination to develop diagnostic hypotheses, and then use site-of-lesion laboratory tests to confirm or rule out their hypotheses. In designing effective rehabilitation exercise programs, in contrast, clinicians require additional knowledge of the subject's functional impairments and adaptive response capabilities. For this reason, objective tests that isolate and quantify the functional impairments associated with balance disorders complement the information provided by site-of-lesion tests and complete the clinical information necessary for effective treatment planning and outcome documentation.
To develop methods and devices for isolating and quantifying functional impairments of the balance system, it is first necessary to understand the functional organization of the balance system. The balance system includes a number of processes that can be grouped into distinct but interdependent systems—one responsible for gaze stabilization and the other responsible for postural stabilization. The gaze stabilization system maintains the gaze direction of the eyes relative to surrounding visual targets as the subject actively moves within his or her environment. Stabilizing the direction of gaze while a person moves maintains their visual acuity during activities involving active head and body movements. When individuals with impaired gaze stabilization participate in activities involving self-motion and moving objects in the surrounds, moving objects can appear blurred while stationary objects can become blurry and sometimes appear to be in motion.
A detailed discussion of gaze stabilization can be found in “Disorders of the Vestibular System” edited by Robert W. Baloh and G. Michael Halmagyi and published by Oxford University Press, New York, in 1996 (Chapters 3 How Does the Vestibulo-ocular Reflex Work?, and Chapter 6, How Does the Visual System Interact with the Vestibulo-ocular Reflex? both of which are hereby incorporated herein by reference). In summary, the gaze stabilization system directs the eyes towards visual objects in the surround through the cooperative interactions of four movement subsystems:
1) The vestibulo-ocular reflex (VOR) system. The VOR is a fast acting system that relies on sensory inputs from angular velocity sensors within the vestibular system (called semicircular canals) to reflexively rotate the eyes in directions that are equal and opposite the rotations of the head. These eye movements are mediated by relatively direct brainstem pathways linking receptors of the semicircular canals with the eye muscles. Hence, VOR movements are fast and able to compensate head movements at frequencies up to 2.0 Hz without needing visual information about object position. Because VOR movements are controlled by inner ear head velocity sensors, they are ineffective at rotation frequencies below 0.1 Hz. Furthermore, the VOR is most accurate in opposing head movements about the yaw axis, i.e. head movements to the left and right of center in the horizontal plane, and is less accurate for pitch axis movements, i.e. up and down movements in the vertical plane.
2) The smooth pursuit eye movement system. The smooth pursuit eye movement system enables individuals to direct their gaze to discrete moving objects within their visual surround. This gaze stabilization system relies on visual position information from the selected object. It is significantly slower than the VOR, because the visual feedback information is mediated by complex pathways involving image processing within the visual cortex. In contrast to the VOR, the smooth pursuit system is effective at the lowest frequencies of head movement and is equally accurate for movements in all directions.
3) The saccadic movement system. The saccadic movement system generates rapid “catch-up” eye movements directed from any given gaze position to the direction of a selected discrete object within an individual's visual surround. Like smooth pursuit movements, saccadic movements require visual position information relative to the selected visual object. In contrast to smooth pursuit movements, saccadic movements are faster acting, but they are limited to discontinuous direction changes rather than smooth continuously controlled movements.
4) The optokinetic movement system. The optokinetic movement system generates smooth eye movements in the direction of continuous, large field motions of the visual surround. Smooth optokinetic movements in the direction of the large field motion are interposed between brief saccadic movements that return the gaze direction back to the original position. In contrast to the smooth pursuit system, optokinetic movements require large fields of continuous, uniform visual surround motion. In the absence of a discrete target, the optokinetic system can stabilize the eyes on a large field visual surround for brief intervals of time. When a discrete target and its surrounding background field are moving together, the optokinetic system can assist the smooth pursuit system in tracking the discrete object. On the other hand, when a discrete target and its large field background move differently, the smooth pursuit and optokinetic systems can potentially interfere with one another.
In addition to the above four eye movement control systems, there are reflexive, automatic and voluntary motor systems for moving the head relative to the body. These movement systems provide additional assistance in maintaining the stability of gaze on visual objects within the surrounds. Automatic compensatory head movements that accompany automatic stabilizing postural movements during upright standing are examples of head movements that assist in gaze stabilization. As described by Nashner L M, Shupert C L, Horak F B. Head-trunk movement coordination in the standing posture in Chapter 21 of Pompeiano O, Allum J H J, eds. (1988) Vestibulospinal Control of Posture and Locomotion, Progress in Brain Research, Vol 76. Amsderdam Elsevier Science Publishers, pp. 243–251 (which is hereby incorporated by reference), automatic postural responses that sway the body backwards and forwards about the ankles and hips are coordinated with automatic head-neck movements that pitch the head in the opposing direction, thereby helping to maintain the angular orientation of the head relative to the visual surrounds.
During daily life activities, cooperative interactions among the VOR, smooth pursuit, saccadic, and optokinetic eye movement systems, as well as between the postural and head-neck movement systems, allow individuals to maintain their direction of gaze on selected visual objects in their surrounds while performing motor tasks under a wide variety of conditions. When the head is moving and the selected visual target is fixed, the VOR system stabilizes the direction of gaze during more rapid movements while the smooth pursuit system, assisted by the optokinetic system under some conditions, provides gaze stability relative to the slower head movements. When the head is fixed and objects in the visual surround are moving, the smooth pursuit system, again assisted by the optokinetic system under some conditions, stabilizes gaze on objects that are moving slowly. When surrounding objects move more rapidly, the smooth pursuit system cannot maintain gaze stability, and “catch-up” saccadic eye movements are used to re-stabilize the gaze. When the body sways during standing, moves up and down, moves from side to side, and/or tilts forward and backward during locomotion, coordination of postural and head-neck movements help maintain the angular orientation of the head relative to the visual surrounds.
When individuals suffer pathological changes in one or more of the four eye movement systems, changes in the adaptive interactions among the four systems may compensate for some of the resulting gaze problems, while other gaze stabilization problems may persist regardless of any adaptive changes. Some subjects with defects in the VOR system, for example, attempt to deliberately limit their activities to slower head movements and substitute smooth pursuit movements to stabilize their gaze on fixed objects. When forced to make more rapid head movements, these individuals may use catch-up saccades that, at best, provide only intermittent gaze stability.
To effectively plan courses of treatment for individuals with impaired gaze stability, the clinical evaluation should provide the following: 1) isolation and quantification of the impairments to gaze stabilization; 2) identification of impairments that can be ameliorated or eliminated by medical and/or rehabilitation therapies; and 3) identification of adaptive strategies that will result in the best visual acuity function relative to the subject's lifestyle demands. Due to wide variations in the relations between pathological and functional mechanisms among subjects, clinicians desiring to improve a subject's gaze control function require information not only of the underlying pathologies but also of the impairments affecting the four control systems and their adaptive interactions.
Because systems for maintaining postural and gaze stability share visual, vestibular, and proprioceptive sources of orientation information and the systems for controlling body and head-neck stabilizing movements are coordinated, subjects with pathological changes in systems for maintaining postural stability may also experience problems with gaze stability and visual acuity. Therefore, to effectively plan courses of treatment for individuals with impaired gaze stability, it is frequently necessary to isolate and quantify additional impairment information related to interactions among the systems for postural and head-neck stabilization.
There are observational and computerized objective tests within the prior art for evaluating individual components the gaze stabilization system and for testing an individual's visual acuity while moving:
1) The VOR System: Rotary chair systems manufactured by several companies, are considered standard methods for quantifying the physiological characteristics of the VOR system. A subject is harnessed into a chair with his or her head fixed and the room darkened. The chair is rotated about a fixed vertical axis under computer control while movement responses of the eyes are recorded using either electrical or infrared optical methods. The computer controls the frequency the chair rotations, records the resulting eye movements, and then correlates the two quantities to determine the gain, phase, and directional preponderance relationships between head and eye movements across a spectrum of frequencies. A detailed description of rotational chair testing of the VOR, including additional scientific and clinical references, can be found in Chapter 6 entitled “Rotational Chair Testing” in: Practical Management of the Balance Disorder Patient by Neil T. Shepard and Steven A. Telian (1996) Singular Publishing Group, Inc. San Diego pp. 221 (which is incorporated herein by reference).
2) Saccadic System. Eye movement systems manufactured by several companies including ICS Medical of Shaumberg, Ill., and Micromedical Systems of Springfiled Ill. quantify the physiological characteristics of the saccadic eye movement system. A subject sits in a chair and views a light bar display on which discrete targets are displayed under computer control while his or her eye movements are measured using either electrical or infrared optical methods and recorded by the computer. The subject is instructed to track targets that jump suddenly from one position to another on the light bar display. The computer then correlates the movements of the subject's eyes relative to the timing, direction, and amplitude of target movements to quantify the latency, velocity, and accuracy of the resulting saccadic eye movements. A detailed description of saccadic eye movement testing, including additional scientific and clinical references, can be found in Chapter 4 entitled “Electronystagmography Evaluation” in: Practical Management of the Balance Disorder Patient by Neil T. Shepard and Steven A. Telian (1996) Singular Publishing Group, Inc. San Diego pp. 221 (which is incorporated herein by reference).
3) Smooth Pursuit System. This movement system is typically tested using either observational methods or recorded eye movements. During observational testing, a subject follows the clinician's finger while the clinician observes the eye movements. The clinician observes whether the subject's eyes move smoothly and in conjunction (together), in a series of small jerks, or disconjugately. Jerky and/or disconjugate eye movements are indicative of failure of the smooth pursuit system. Alternatively, the subject's pursuit eye movements may be electrically or optically recorded and the degree of smoothness evaluated by analyzing the smooth pursuit “velocity gain”, the speed of the eye movement compared to the speed of the target movement, and by visually inspecting the recorded eye movement traces for evidence of small movement jerks. A detailed description of smooth pursuit eye movement testing, including additional scientific and clinical references, can be found in Chapter 4 entitled “Electronystagmography Evaluation” in: Practical Management of the Balance Disorder Patient by Neil T. Shepard and Steven A. Telian (1996) Singular Publishing Group, Inc. San Diego pp. 221 (which is incorporated herein by reference).
4) Optokinetic System. The optokinetic system can also be tested observationally or with recorded eye movements. During either type of testing, a subject views a large field surface with high contrast, alternating stripes that move continuously in a direction perpendicular to the orientation of the strips. The intensity of the subject's optokinetic eye movements relative to the velocity of the large field surface movement, slow in the direction of stripe movement and rapid in the opposite direction, are then documented.
5) Dynamic Visual Acuity. Differences in a subject's visual acuity with the head fixed and head moving can be quantified using observational methods or computerized devices manufactured by several companies including NeuroCom International, Inc. of Clackamas, Ore., and Micromedial of Springfield, Ill. Observational tests of dynamic visual acuity (DVA) are based on the standard Snellen eye chart. As described by Demer J L, Honrubia V, Baloh R W (1994) “Dynamic visual acuity: a test for oscillopsia and vestibule-ocular reflex function” American Journal of Otology 15: 340–347 (herein incorporated by reference), visual acuity is assessed, first with the head fixed and second with the subject moving the head back and forth at a pace instructed by the administrator of the test. By observing the subject, the test administrator verifies that the subject continues moving the head while reading the Snellen chart. The DVA results are reported as the number of Snellen lines of acuity decrement observed during the head moving as compared to head fixed conditions. Another form of eye chart that may be employed is the so-called “Tumpling E”. In accordance with tests using the Tumpling E, a subject views a letter “E” of a specified optotype size and orientation and is asked to correctly identify the orientation of the “E” (up, down, left, right).
The observational DVA test has several problems that limit the value of the information in discriminating among the various causes for loss of gaze stability: (1 results are approximate, because the administrator can only estimate the velocity of the subject's head movements; and (2 some subjects can compensate for losses within the VOR stabilization system by very briefly slowing their head movements for small fractions of a second and using catch-up saccadic movements to stabilize gaze on the chart.
In computerized versions of the dynamic visual acuity test, visual images are generated and displayed by computer, subjects wear sensors on the head that quantify head movement velocities, and the computer receives the head movement information and controls the presentation times of visual images such that they appear only when the head is moving in accordance with instructions. These methods prevent subjects from compensating with catch-up saccadic movements and also allow precise specification and verification of the head movement velocity. A description of computerized dynamic visual acuity testing can be found in Herdman, S, Tusa R, Blatt P, Suzuki A, Venuto P J, Robert D (1998) “Computerized dynamic visual acuity test in the assessment of vestibular deficits.” American Journal of Otology 19:790–796 (which is hereby incorporated herein by reference).
There are also observational and computerized objective tests for evaluating individual components of postural stability while an individual stands freely and/or walks:
1) One commonly used observational clinical test for assessing an individual's postural stability was described by Koles Z J, Castelein R D (1980) “The relationship between body sway and foot pressure in normal man” Journal of Medical Engineering and Technology 4: 279–285 (which is also incorporated herein by reference).
2) A number of manufacturers including NeuroCom International, Inc. of Clackamas, Ore., and Micromedical Technologies of Springfield, Ill., manufacture simple postural stability assessment devices utilizing fixed forceplate measuring devices, as well as forceplates suspended on compliant devices, linked to computers. These devices quantify an individual's postural sway while standing quietly and during attempts to shift the body weight.
3) In previously issued patents related to sensory integration and movement coordination analysis series, apparatus and methods for characterizing gait, etc., (including U.S. Pat. Nos. 4,738,269, 4,830,024, 5,052,406, 5,303,715, 5,474,087 and 5,697,791—all incorporated herein by reference) the present inventor has described methods and devices for measuring the postural stability of individuals standing and walking while subjected to perturbations generated by controlled movements of the support surface. Some of these methods and devices have been incorporated into the EquiTest® system manufactured and marketed by NeuroCom International, Inc. of Clackamas, Ore.
It is generally understood that the vestibular function and eye movement control information provided by rotary chair, smooth pursuit, saccadic, and optokinetic tests, as well as other “site of lesion” otologic and neurological tests of vestibular and motor system physiological functions do not correlate well with the symptoms and functional impairments experienced during daily life activities by subjects with balance system problems. This is because gaze and postural stabilization systems are highly adaptive, and individual subjects with similar pathologies use their residual VOR, smooth pursuit, saccadic, optokinetic, and postural stability capabilities differently, resulting in different symptoms and functional problems. The DVA test, in contrast, does quantify how well a subject can accurately perceive fixed visual objects while moving the head. For reasons described above, however, the DVA test alone does not isolate and quantify impairments related to gaze stabilization while individuals attempt to view: (1) fixed visual targets while moving the head, (2) moving targets with the head fixed, (3) fixed or moving targets while maintaining free standing and walking balance, and (4) combinations of these task conditions.
During daily life activities, such as driving a car or participating in recreational sports activities, individuals are frequently required to observe stationary and moving visual objects while they themselves are actively moving. The ability to differentiate among a variety of impairments contributing to poor gaze stability and visual acuity during these types of daily life activities would have significant clinical value. In the majority of subjects with balance system problems affecting gaze stability, pharmacological and surgical treatments designed to stabilize or reduce the impact of disease processes are guided by information provided by physiological “site of lesion” tests. While medical treatments can sometimes stabilize the underlying patho-physiological processes and thereby slow the progression of disease, such treatments seldom resolve the underlying pathology or eliminate a subject's functional gaze and postural stability problems. Rehabilitation treatments, in contrast, can substantially improve gaze and postural stability functions and reduce adverse symptoms in a majority of subjects. In contrast to medical treatment planning, however, isolating and quantifying impairments provides the information necessary to focus training exercises on those impairments having the greatest negative impact on the subject's daily life activities. Impairment information also provides objective benchmark against which treatment results can be documented.
Embodiments of the present invention relate to new methods and devices for isolating and quantifying functional impairments among the VOR, smooth pursuit, saccadic, optokinetic eye movement, postural stability systems, and their adaptive interactions.